This invention relates generally to the prevention of corrosion of refractory and metal surfaces. More particularly, this invention has special application to incinerators and a method of inhibiting corrosion of incinerator elements by introducing additives to sewage and waste products burned therein.
Incineration of sewage and other waste materials including combustible chemicals and oils is used in many places where other forms of disposal are precluded. Generally, incineration of wastes allows the advantageous use of relatively compact, lightweight treatment units and greatly reduces the volume and weight of the end products. Thus, incinerators find particular application in ships and marine vessels. As opposed to land based incinerators, which are normally designed to burn a different character of waste products and often contain heavy ceramic refractory linings, shipboard incinerators tend to be smaller, usually have metal lined refractory surfaces to effect weight reduction as exemplified by U.S. Pat. No. 3,861,330 and are often operated at higher liner or wall temperatures than such land based incinerators.
Waste products commonly disposed of in shipboard incinerators include sewage mixed with oily residues, seawater and various other chemical compounds. Normally, the combustion of such products produces ash products which if they undergo partial or complete fusion in the combustion zones become sufficiently soft, plastic or molten enough to adhere to the internal surfaces of the incinerators. Such an occurrence produces results which tend to adversely affect the efficient performance of incinerators. For example, the accumulation of molten ash or slag on heat exchange surfaces tends to reduce the heat absorbing ability thereof and continued accumulation of molten combustion products often clogs various flow paths, thereby reducing the flow and perhaps raising the temperature of the combustion products. Importantly, it was found that combustion products in a molten or liquid state react much more rapidly with interior surfaces of incinerators than when in the solid state and, as a consequence, promote more rapid corrosion. Accordingly, it was desirable to cause the combustion products to exist in a solid phase at the incinerator interface, thereby reducing the reaction rate with the incinerator surfaces without reducing the operating temperatures of the incinerators. This is particularly critical with incinerators designed to burn shipboard wastes, which typically contain sewage having about a 95% water content wherein a minimum operating temperature is normally required to evaporate the water content and burn the waste solids in a relatively short period of time. The chemical structure of the compounds found in the solid waste or fecal matter is unknown in general because of the variable and complex nature of sewage.
The teachings of the following references are primarily concerned with additives for decreasing the deleterious effect of impurities in the fuel oils: British Pat. Nos. 496,692; 728,812; 878,723; Canadian Pat. Nos. 619,117; 714,846; and U.S. Pat. Nos. 2,800,172; 2,844,112. Designed to clean up the fuel oils, the additives are normally introduced directly into the fuel such that reactions with the undesirable impurties in the fuel can occur during the combustion process. In contrast, the process of the present invention relates to the incineration of shipboard wastes consisting essentially of about 95% water and about 5% solids of chemically complex constituents, wherein the additives are intended to react therewith. In view of the limited reaction times in the present process, as short as one second with some incinerators, it can be appreciated that proper combustion of the waste solids requires controlled dispersion of the waste mixture into the combustion zone so that the additives remain in contact with the waste solids, wherein reactions between the additives and the impurities in the fuel are of negligible importance.
Additionally, the following references generally relate to fluidized bed processes wherein the chemical reaction time is often relatively long as compared with many incineration processes; U.S. Pat. Nos. 3,881,430; 3,888,193; 3,888,194; 3,921,543 and 4,060,041. In view such relatively long contact times, additives are commonly introduced into the fluidized bed materials to prevent such bed materials from sticking together and sintering, to remove noxious and toxic gasses by reaction therewith and to act as reaction catalysts; but such additives are not primarily intended to inhibit or prevent corrosion of the walls of the reaction chamber. As aforementioned, the process of the present invention is designed for the combustion of wastes having a high water content wherein the reaction period is relatively brief.
Examples of incinerator structures are generally disclosed in U.S. Pat. Nos. 3,805,714; 3,861,330; 3,892,190 and 4,002,147. In the incineration of aqueous waste containing organic material and other solids, it is preferred that such solid waste material is finely dispersed and suspended in a liquid slurry prior to introduction into the combustion zone. Common means for introducing the liquid waste slurry into the combustion zone includes conventional conduits and liquid lines with pressure sources connected thereto such that the waste slurry is directed through injection nozzles of various design. Upon introduction into the combustion zone, the sudden expansion of the waste slurry into a larger volume produces a relatively large effective reaction surface for rapid evaporation of the water and combustion of the waste solids. The turbulence created by the injection of the waste slurry into the combustion zones and the directional flow pattern of gasses in the combustion zones promotes rapid incineration of the waste solids. This is particularly relevant in view of the fact that the reaction and retention periods of waste products in the combustion zones of some incinerators is on the order of a few seconds. Customarily, the operating temperatures of the walls of the incinerators are lower than the flame or combustion temperatures occurring in the combustion zones of the incinerators.